1. Field of the Invention
The present invention relates to an optical image pickup system and more particularly to an optical system of a color television camera using a stripe-shaped color separation filter.
2. Description of the Prior Art
In general, in a single or dual tube type color television camera including a stripe-shaped color separation filter, an image bearing light from an object is separated into components of red, blue and etc., by the stripe-shaped color separation filter, and the separated light is directed onto a photo conductive film which is, in turn, scanned by an electron beam radiated from an electron gun to produce video signals corresponding to the respective color components from a signal electrode.
When the image of the object is formed on the stripe-shaped color separation filter, if the image of the object includes a frequency component which is identical to the spatial frequency of the stripe-shaped color separation filter, a chrominace signal of the image of the object may be erroneously taken independently of the color inherent in the object due to crosstalk between a luminance signal of said frequency component which is derived from a photo-electro conversion plate of the camera tube and the chrominance signal of the image of the object, because in the television camera of this type a pattern of the stripe-shaped color separation filter is projected on the photo-electric conversion plate so that the pattern of the stripe-shaped color separation filter is substantially superimposed on the object image formed on the photo-electro conversion plate. In other words, the object image is taken as a spurious color signal so that the reproduced image is miscolored. This phenomenon is observed in a single tube type or dual-tube type color television camera using the stripe-shaped color separation filter and is particularly remarkable in the single tube type.
Various approaches have been proposed to resolve the above problem, typical ones of which are given below.
First, it has been proposed to arrange a double refraction plate in the image pickup path of the television camera which uses the stripe-shaped color separation filter, for producing a so-called optically defocussed image for the spatial frequency component corresponding to the spatial frequency of the stripe-shaped color separation filter, while maintaining as high a sharpness of image as possible for the spatial frequency band (hereinafter referred to as the luminance band) which is lower than the frequency band to which said spatial frequency belongs (hereinafter referred to as the chromaticity band). The double refraction plate which forms an optical low pass filter by making use of the double refraction property thereof may be a quartz plate, a calcite plate or a plate of LiNbO.sub.3, which divides an incident ray into two optical paths for abnormal rays and normal rays to attain the above object. A curve A shown in FIG. 1 represents an optical transfer function or a response function (MTF) R(f) (hereinafter referred to as the response function) obtained by this method, in which R(f).sub.A which means the R(f) for the curve A is given by the following equation; ##EQU1## where f is the spatial frequency, and fo is a trap frequency which results in R(f).sub.A = 0. The trap frequency fo is determined by indexes of refraction for the normal ray and abnormal ray of the double refraction plate, angle of crystal cut with respect to crystal axis of the plate and the thickness thereof. For the purpose of supressing the spurious color signal of frequency fo is set between the frequencies f.sub.1 and f.sub.2 in the chromaticity band.
As seen from the curve A of FIG. 1 and the equation (1) above, this type of optical system has a relatively high resolution power in the luminance band resulting in a sharp image, but presents a relatively small reduction of spurous signals because the trap frequency fo which causes R(f).sub.A = 0 usually lies within the color signal carrier band f.sub.1 .about. f.sub.2 and the sharpness of image abruptly increases for the frequencies above and below the frequency fo. In a two-frequency separation type single tube color television camera system which has a relatively broad chromaticity band such as 3.0 MHz - 6.5 MHz, the reduction of the spurious signal tends to be insufficient.
Secondly, a diffraction grating spatial frequency filter system has been proposed in which the spurious signal is suppressed by making use of the light diffraction effect by a brightness grating or a phase grating. As the grid arrangement of the grating, one-dimensional or two-dimensional, square wave or sine wave phase grating array, or a random distribution grating is frequently used. The curve B in FIG. 1 shows a response function (MTF) R(f) obtained when the filter comprising a square wave phase grating is used. R(f).sub.B represents the function R(f) for the curve B, and the following equations are given for the curve B; EQU fa = a/.lambda.b ##EQU2## When 0.ltoreq.f.ltoreq.f.sub.Q, ##EQU3## When f.sub.Q .ltoreq.f.ltoreq.f.sub.p, EQU R(f).sub.B = Q (3)
where
a: width of a portion which causes phase retardation .delta. of the grating PA1 b: converted distance between image planes of the grating, PA1 X: period of the grating, PA1 .delta.: phase retardation of a wave plane by the phase grating, which is given by ##EQU4## .lambda.: light wavelength n: index of refraction of the phase grating forming material PA1 n': index of refraction of medium PA1 d: thickness of the phase grating PA1 fa: cutoff frequency PA1 f.sub.p : frequency at which the response function starts to rise again.
According to this optical system, as seen from FIG. 1 and the equations (2) and (3) above, although substantially sufficient reduction of the spurious color signals can be attained over the entire range of the chromaticity band by design approach, when fa is set equal to fo, sufficient resolution power cannot be attained in the luminance band having a lower frequency.